1. Technical Field
The present invention relates generally to perforating a well bore with a plurality of perforating guns. More specifically, the present invention relates to an improved apparatus for transferring the detonating reaction from one gun to the next gun without interruptions.
2. Description of the Related Art
Well bore or down hole perforating guns can be conveyed into a well bore in one of two methods--first by using a wireline for conveying the perforating gun, including slickline (conductorless wireline); and second, by using tubing for conveying the perforating gun. In the wireline conveyance method, a hollow carrier gun is attached to the wireline by means of a perforating head. A wireline is a cable which contains at least one electrical conductor. The gun is lowered into the well bore and aligned across the geologic zone of interest and electrically detonated from the surface. Wireline perforating is generally limited to combinations of guns totaling 30 or 40 feet in length due to the weight of the guns on the wireline.
The second conveyance method is by tubing conveying perforating (TCP) guns. Conveying perforating guns into a well bore using tubing generally requires a much longer time for descent to the geologic zone of interest. Once at the zone of interest, the guns are again aligned and detonated. Detonation is initiated either by percussion (i.e., by dropping a bar or weight through the tubing striking the detonation head of the perforating gun); pressure (i.e., by increasing the hydrostatic pressure within the tubing until it reaches a predetermined pressure on the perforation head, at which time a shear pin is broken and the perforating gun is detonated); or electrically (i.e., by lowering a wireline through the tubing which connects to the perforating head of the gun and detonating the perforating gun by passing an electrical current through the wireline to the perforating head).
There are two primary advantages of tubing conveying perforating guns over wireline conveying perforating guns. First, the length of the guns in the tubing conveying method can be much longer because tubing supports a much higher gun weight than wireline. Second, the well bore need not be vertical but might, instead, be highly deviated or even horizontal; and the tubing conveying perforating guns can still be deployed across the geologic zone of interest.
Wireline conveying perforating guns, on the other hand, have the advantage of reduced trip time in and out of the well bore. For instance, a thin geologic zone of interest which requires one run or trip in the well bore, where the well bore is substantially vertical, takes much less trip time than using tubing as a conveyance. A wireline conveying perforating gun can be lowered into the well bore, the casing perforated, and the spent perforating gun retrieved out of the well bore in less time than it would take to lower the tubing conveying perforating gun. As opposed to using wireline, it takes much longer to run lengths of tubing into the well bore by connecting one tubing stand or joint at a time and positioning detonating the tubing conveying perforating gun either hydraulically, mechanically or electrically.
However, because the maximum perforating gun weight for tubing is much higher than for wireline, one trip into the well bore using tubing might yield a perforated interval equivalent to many wireline trips into the well bore. Also, perforating horizontal and highly deviated wells may require tubing as a means to move the perforating gun because the perforating gun will no longer fall with gravity due to the amount of deviation in the well bore.
Generally, temperature increases with well depth, and well bore temperatures of 400.degree. F. to 450.degree. F. are not uncommon. The explosive components of a perforating gun are particularly susceptible to the effects of high temperatures in well bores. At temperatures above the rating of a particular explosive component, detonation results become unpredictable. A high order detonation (i.e., one in which the explosive reacts at an extremely rapid rate, generating a simultaneous pressure wave) is necessary for an explosive charge to perforate a well bore, and it is less likely in cases where the well bore temperature exceeds the temperature rating of the explosives. Another factor which reduces the likelihood of a high order detonation is the amount of time the explosive is exposed to certain temperatures. Explosives are rated by time limit and maximum temperature. Generally, the higher the temperature, the less time the explosive can tolerate that temperature and still predictably detonate at high order. FIG. 1 illustrates a typical time/temperature chart for explosives used in perforating guns. 2,6-bis(Picrylamino)-3,5-dinitropyridine (PYX), hexanitrostilbene (HNS), cyclotetrimethylenetetrinitramine (HMX), and cyclotrimethylenetrinitramine (RDX) are common explosives used in the oil well perforating industry. Note from operating curve 10, that PYX is rated at 450.degree. F. for 1000 hours, but the time at 550.degree. F. drops to 10 hours. The time/temperature operating curves for PYX 10, HNS 20, HMX 30 and RDX 40 are roughly parallel through different operational temperatures.
FIG. 2 illustrates the components in a typical perforating gun system. A conventional perforating gun consists of three explosive components. The first, used at the initiating stage, is the blasting cap or detonator (not shown). Detonators can be either electric or percussion. The blasting cap initiates the detonation that is transferred through detonating cord 102 (second component) to individual shape charges 104 (third component). The detonating cord may be run from one end of the gun to the other and provides an explosive shock wave of sufficient force to detonate each shape charge. Shape charges are usually oriented perpendicular from the axial line of the gun. The detonator cord runs behind each shape charge and triggers a small primer charge (not shown) in each individual shape charge 104 when detonated.
In instances where the interval to be perforated is longer than any one perforating gun length, perforating guns must be combined in order to obtain the proper length. In wireline operations, each gun can be configured with its own blasting cap and fired sequentially from the bottom up. However, there are instances in which two perforating guns are detonated simultaneously. Two guns, top or upper perforating gun 106 and bottom or lower perforating gun 108 can be joined with a gun head adapter or transfer sub which consists of two short, threaded stubs of steel crossover pin-and-box 110 and grooved tandem 120. Detonating cord 102 fastens through crossover pin-and-box 110 and grooved tandem 120, passing from the upper perforating gun 106 to the lower perforating gun 108. Detonating cord 102 is usually separated at the makeup point between crossover pin-and-box 110 and grooved tandem 120. Thus, the length of the gun can be divided into two smaller guns, upper perforating gun 106 and lower perforating gun 108, and re-combined at the well site by inserting grooved tandem 120 into crossover pin-and-box 110. The detonation, therefore, must propagate from detonating cord 102 in upper perforating gun 106 to detonating cord 102 in lower perforating gun 108.
In order to facilitate the propagation of the detonation from upper perforating gun 106 to lower perforating gun 108, or visa versa, each end of detonating cord 102 is usually terminated with a bi-directional booster at both crossover pin-and-box 110 and grooved tandem 120. Upper bi-directional booster 116 is positioned by upper booster sleeve 126 in upper gun 106 and lower bi-directional booster 118 is positioned by lower booster sleeve 128 in lower gun 108. A bi-directional booster provides an extra boost of explosive force, propelling the explosive shock wave from the first booster toward the second booster, which then reinitiates the detonating cord on the second booster's side of the transfer sub. Simultaneous detonation of multiple perforating guns is more popular with TCP.
In TCP applications, each gun is usually brought to the well with a fixed number of individual shape charges 104, or shots; and detonating cord 102 at the connecting point between each upper gun 106 and lower gun 108 is terminated with bi-directional boosters 116 and 118, respectively, inserted inside grooved tandem 120 into crossover pin-and-box 110. The distance or gap between bi-directional booster 116 and bi-directional booster 118 is critical. When the guns are fully joined, the distance between the two bi-directional boosters should be approximately one-quarter of an inch. However, at the time when the individual guns are loaded, either at the shop or at the well site, the booster gap is approximately one-eighth of an inch from the face of either crossover pin-and-box 110 or the face of grooved tandem 120. When grooved tandem 120 is screwed into crossover pin-and-box 110, upper booster 116 is approximately one-fourth of an inch from lower booster 118. Thus, the booster gap is approximately one-fourth of an inch. At the time the detonating cord is detonated, the detonation force easily propagates across the one-fourth of an inch gap between the two transfer subs.
As discussed above with respect to FIG. 1, explosives are rated by the time and temperature at which they are considered reliable. For instance, PYX explosive has a temperature-time rating of 450.degree. F. for 1000 hours, while HMX has a temperature-time rating of 400.degree. F. for one hour. Time-temperature rating is an extremely important consideration when choosing which explosive to use in a particular perforating application. For example, a well bore is known to be 400.degree. F. at the geologic zone of interest. Assuming trip time would be over two hours to convey the perforating guns to the geologic zone of interest, align the guns across the geologic zone of interest and detonate the guns, an explosive with a temperature-time rating of 450.degree. F. for one hour, (such as HMX), would be an inappropriate choice. For this well bore, the time required to perforate at the down hole temperature would take the explosive over its temperature rating. Therefore, the most appropriate explosive for this well would be one with a higher temperature-time rating than the temperature known to exist in the well, such as HNS or PYX.
A problem associated with perforating high temperature wells is that of detonating cord shrinkage. The detonating cord may shrink substantially in length even while exposed only to times and temperatures below the rating of the explosive. Shrinkage or pull-back causes the boosters to be pulled back from each other and, thus, widens the gap between them. This situation leads to unpredictable detonation force transfer between perforating guns. Another problem attributed to shrinkage is that the explosive within the detonating cord is pulled back from the explosive within the bi-directional booster. The results are similar because the detonation force is not transferred to the booster from the detonating cord.